During the “back end of line” (BEOL) portion of fabricating a microelectronic device, several conducting layers are stacked on each other in sequential operations. The conducting layers, which are also referred to as interconnects, are separated by one or more dielectric layers to electrically insulate adjacent interconnects and to prevent undesirable crosstalk between the conducting layers. The interconnects are in the form of vias and lines or may be contacts to source/drain regions in the substrate. There is a significant challenge to improve the reliability of the final device because the repeated passage of current through an electrical circuit stresses the (metal) conductors and may induce voids that cause device failure. Stress induced voiding is a common problem in multi-level interconnects, particularly those comprised of copper. Reliability is an increasing concern as interconnects become smaller and higher current densities are employed. One leading cause of device failure is electromigration that forms a void by the movement of metal ions or vacancies in a conductive element as a result of a current passing through it. A continuing trend is to manufacture devices in which failure mechanisms are substantially delayed or prevented from occurring by incorporating new designs and improved materials.
While operating an integrated circuit device, a current typically flows from a first conducting layer through a diffusion barrier at the bottom of a via and then through the via before reaching a second conducting layer. For example, consider the interconnect structure in FIG. 1 in which a first conducting layer 3 is formed in a first dielectric layer 2 on a substrate 1. Typically, the first conducting layer 3 is coplanar with the first dielectric layer 2. A second dielectric layer 4 is deposited on the first dielectric layer 2 and on the first conducting layer 3. Optionally, the first and second dielectric layers 2, 4 are part of a stack of dielectric layers further comprised of one or more etch stop layer or barrier layers as is appreciated by those skilled in the art.
A conventional patterning and metal deposition (damascene) sequence is used to form a via 5 above the first conducting layer 3 and a second conducting layer 7 aligned above the via 5. Typically, a diffusion barrier layer 6 is deposited in a damascene opening prior to the second metal deposition. A compressive stress builds up on the downstream side of the diffusion barrier layer 6 while a tensile stress increases with time on the opposite side of the diffusion barrier at the bottom of the via 5. There tends to be a movement of metal in the first conducting layer 3 that leads to void 8 formation in locations of tensile stress such as immediately upstream in the current flow from the diffusion barrier 6. Thus, a portion of the first conducting layer that is adjacent to the via 5 is especially susceptible to void formation due to electromigration. Furthermore, a first conducting layer 3 with a large surface area is likely to form a void more readily. A via test structure for monitoring the effect of a first and second conducting layer pattern on void formation is desirable to enable a better understanding of how design and materials may be optimized to provide higher reliability.
Void detection in metallization patterns is accomplished in U.S. Pat. No. 5,504,017 by passing a current across a metal layer to generate a hot spot in a barrier layer adjacent to the void. The hot spot is detected by an infrared technique or by coating a liquid crystalline material on the metal and measuring a calorimetric response.
A test structure is disclosed in U.S. Pat. No. 6,004,827 in which a metal runner is formed on a substrate. After a sintering process, a dielectric layer is removed to reveal bumps on the runner. When bump concentration at a given location is more than 20% higher than the average bump density, long term failure is predicted at that site.
In U.S. Pat. No. 6,498,384, a test structure is fabricated on a semiconductor wafer and comprises a first layer of metal that has second and fourth channels which are connected in series by vias with first, third, and fifth channels in a second metal layer. Openings in a capping layer allow the first and fifth channels to be probed for resistance and compared to a calibration measurement.
U.S. Pat. No. 6,320,391 describes a long narrow test conductor that is connected to an extension metal conductor on each end by a plurality of vias that avoids a current crowding effect when placing only one via at each end of the test conductor. The test structure is compatible with a high stressing current.
In U.S. Pat. No. 6,570,181, a reliability test structure is described as having a chain of a plurality of long test links formed in a first metal layer that are alternately interconnected by a plurality of short links formed in a second metal layer. The long and short links may be arranged in a serpentine configuration.
K. Yoshida, T. Fujimaki, K. Miyamoto, T. Honma, H. Kaneko, H. Nakazawa, and M. Morita describe in “Stress-Induced Voiding Phenomena for an Actual CMOS LSI Interconnects” in Electron Devices Meeting, 2002, IEEE, Vol. 8-11, pages 753 to 756 that a vacancy in a bulk metal is able to diffuse through a wide metal layer or in a delayed fashion through a narrow metal layer before reaching a via that is connected to the metal layer. Over time, enough vacancies accumulate to form a void at the via bottom. For example, a void occurs more rapidly below the via 51 formed on a wide metal layer 50 in FIG. 8a than below a via 51 formed on an extension 52 from a wide metal layer 50 in FIG. 8b. 
Therefore when performing reliability testing on an interconnect, conventional test structures typically underestimate the lifetime before device failure occurs because they fail to account for voids in a bulk metal such as a bonding pad or a large area metal layer that diffuse through a narrow metal connection to a via. A top-down view of a conventional four port Kelvin test structure 10 for performing resistance measurements is pictured in FIG. 2. A first metal layer comprised of a bond pad 11, a bond pad 12, and metal lines 13, 14 is formed on a semiconductor substrate (not shown). A second metal layer comprised of bond pad 15, bond pad 16, and metal lines 17, 18 is formed above the first metal layer and is separated from the first metal layer by a dielectric layer 19. The first metal lines 13, 14 are connected to the bottom of a via 20 while second metal lines 17, 18 are connected to the top of the via 20. A current of about 1 milliamp (ma), for example, is applied at bond pad 11 and a voltage V1 is measured at bond pad 12 and a voltage V2 is measured at bond pad 16. Bond pad 15 is grounded to zero volts. The resistance R of via 20 is determined as R=(V1−V2)/1 ma.
A reliability test is conducted by heating the substrate with the test structure at an elevated temperature of about 200° C. for a duration of time that may encompass hundreds of hours. Periodic resistance measurements are taken during the period that the substrate is heated and these measurements are compared with the original resistance value. Unfortunately, the reliability test underestimates the time before device failure since vacancies (not shown) in bond pad 11 migrate through metal line 13 to form a void (not shown) at the bottom of via 20 that increases via resistance R to an unacceptable level. Additionally, vacancies in bond pad 12 may migrate through metal line 14 to increase the rate of void formation at the bottom of via 20. It is understood that the dielectric layer 19 is typically a stack of dielectric layers which are not shown in order to simplify the drawing. Furthermore, first and second metal layers are typically enclosed within one or more diffusion barrier layers that are not pictured.
An improved test structure is needed that prevents vacancies in a bonding pad from interfering with the rate of void formation at the bottom of a via and thereby enables a more accurate determination of the via lifetime when device failure is expected to occur.
A commonly used method to offset the tendency for void formation in an interconnect is to apply a design rule that allows more vias to be connected to large area metal layers. However, this intended remedy has its own drawbacks since this type of design will lead to a higher total resistance and more risk in the via manufacturing process. The practice of adding more vias also increases the difficulty in checking the design rule.
In U.S. Pat. No. 5,614,764, an endcap reservoir is added to an interconnect line for extending electromigration lifetime and preventing void formation. However, the reservoir is not placed between the interconnect line and an adjoining via and therefore does not prevent vacancies from migrating within the interconnect to the via.
An alternative approach to mitigating the effect of electromigration is described in U.S. Pat. No. 6,306,732 in which an imperfect barrier layer is employed at the bottom of a via with a stronger barrier at all other portions of the via. Unfortunately, the problem of vacancy accumulation on the upstream side of the via is not prevented. Thus, an improved interconnect structure is required that overcomes the driving force for vacancies in a large area metal layer to migrate to the upstream side of a via and form a void that degrades device performance.